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Abstract
A novel mode-hop-free (MHF) tunable external cavity diode laser (ECDL) is demonstrated without mechanical pivot-point tuning. By corotating a periscope with an etalon and a narrow band pass (NBP) filter inside an external cavity, the cavity single longitudinal mode selection can be maintained, and continuous tuning can be achieved with optimal synchronization. A MHF continuous tuning range of 1.7 THz has been achieved with a semiconductor gain chip at the wavelength of 652 ± 2 nm experimentally, and the theoretical tuning range can reach over 4.8 THz. The laser linewidth is estimated to be less than 1 MHz (FWHM) by a scanning Fabry-Perot (F-P) and a Michelson interferometer. 1 mW output power with variation of less than 10% in the tuning region of roughly 1.7 THz has been demonstrated.
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1. Introduction
Semiconductor based lasers have a broad gain spectrum in the visible and near infrared portions of the electromagnetic spectrum, which have been widely used in many tuning mechanisms with properties such as MHF, narrow linewidth, wide spectral coverage, continuous tunability, and relatively low cost [1–3]. There are several methods on the market that can achieve such operations, such as temperature control on distributed feedback laser (DFB) [4–6], electronic control on sampled grating distributed Bragg reflector (SG-DBR) [7,8], micro-electromechanical systems (MEMS) control on vertical cavity surface emitting laser (VCSEL) [9,10], F-P and silicon photonic vernier filters [11,12], and diffraction gratings based Littrow and Littman-Metcalf configurations with mechanical pivot point [13–15], electro-optic crystal deflector [16], MEMS reflector [17], and polygon-scanner [18].
The early DFB, DBR, and SG-DBR and superstructure grating DBRs (SSG DBR) [19,20] achieve frequency tuning by changing the current or temperature of the diode. However, the wavelength can only be shifted under a limited spectral range, and the tuning range may not be continuous. Optically and electrically pumped MEMS-VCSEL with narrow cavity (micrometer regimes) can achieve absolute single mode operation through one full free spectral range (FSR) which is typically larger than 100 nm at near infrared. Besides, more than several hundred kHz MEMS resonances can also be achieved through careful design of the membrane thickness, layer stress, and actuator shape [21]. However, as the short cavity length and large FSR, the laser linewidth smaller than ∼MHz is hardly achieved, which is important for many applications such as frequency scanning interferometry (FSI) [22]. Furthermore, the cost and reliability outline significant drawbacks due to extremely precise manufacturing and maintenance down to the nanometer level.
Unlike the MEMS-VCSEL devices, Fabry-Perot tunable filters (FP-TF) based ECDL provides large FSR and narrow linewidth simultaneously and is more compact to achieve higher tuning speed. Its mode selective filter is an independent unit which can be inserted in the longer external cavity to allow ultra-narrow linewidth and wide MHF tuning. One of the two end facets can be a curved mirror on membrane which is able to move precisely and rapidly. Due to the short distance between two mirrors and highly reflective coating, the finesse can be larger than 1000, providing large FSR and narrow linewidth simultaneously. Fiber coupled FP-TF and electrostatic MEMS based FP-TF have been successfully applied on the swept source optical coherence tomography (SS-OCT) [23] and Fourier-domain mode-locked (FDML) lasers [24].
The diffractive grating based tunable ECDL has been commercialized for many years and widely applied, such as spectroscopy [25–27], sensing [28–30], and laser metrology [31–33]. Narrow linewidth, broad tunability, and MHF continuous tuning can be achieved based on the well-known Littrow or Littman/Metcalf configuration, using its scanned grating with rotating polygon mirrors provides wide tuning range, high speed, and narrow linewidth [18]. Electro-optical crystal based deflector is usually used to change the refraction index proportionally with a changing electric field to achieve the large angle deflection on the diffraction gratings [34]. Mechanical tuning is very attractive due to its optical properties including single mode operation, ultra-narrow linewidth and wide MHF tuning range. It works simply based on diffraction grating tuning schemes. In specific, the MHF tuning is achieved by rotating an optical component such as grating, mirror or applying a piezo-electric actuator motion over an optimum pivot point [35–38] while the external cavity frequency keeps in synchrony with the diffracted light [39–42]. The disadvantage in these schemes is that any translation of the optics larger than a quarter of the laser wavelength outside the plane of the optics will destroy the synchronization and cause mode hops. This not only increases the cost of production and maintenance but also reduces the durability because mechanical parts degrade over time and with continuous use. Thus, it is difficult to achieve fast tuning, high repetition, and good reproducibility with continuous movement over the pivot point. Through the advancement of MEMS, the grating based ECDL has seen significant improvement providing smaller, faster, and more accurate devices [43–46]. However, to achieve the broadband MHF tuning, they still rely on the pivot-point-based movement.
In this study, we present a novel ECDL concept based on a mechanical tuning method of the pivot-point-independent design. The synchronous tuning components involve an etalon with a NBP filter and a periscope to guarantee the MHF tuning of the ECDL. The single longitudinal cavity mode can be tuned by rotating the periscope, while the mode selection and mode lock can be achieved by using the synchronized etalon and NBP filter. Unlike the pivot-point based grating tuning, our synchronization only depends on the incident angle which is insensitive to the displacement of the moving elements. Experimentally, a MHF continuous tuning range of 1.7 THz at the central wavelength of 652 nm is obtained and the laser linewidth is estimated to be less than 1 MHz. This new design has the possibility to achieve very large MHF tuning range over 4.8 THz theoretically and reveal a new method to potentially improve the tuning speed and service life with very low cost.
2. Laser cavity design and synchronization analysis
There are some early Littrow cavity designs in which an etalon can be inserted into the cavity to force the cavity into operating as a single mode. However, the tuning process is complicated because the etalon has to be tuned synchronously with the grating and cavity length [47]. Recently, a multi-layered dielectric interference tunable optical filter and an SiO2 plate are synchronized in the ECDL cavity to achieve MHF tuning with range of 8 nm at C band [48], but two components need to be operated separately and simultaneously using a digital signal processing (DSP). A new idea of corotating a glass block and an etalon as the wavelength-selective element inside an external cavity is initially proposed [49]. Although this scheme has an advantage of insensitivity to transition movement, it only gives a theoretical MHF tuning range from ∼250 GHz to ∼320 GHz for an angle of incidence within the etalon of 8° and 10° respectively [50].
In our laser cavity design, instead of gratings, an angle tunable etalon is adopted as a wavelength selective element to achieve MHF tuning, the cavity longitudinal modes must be shifted at the same rate as the etalon peak so that the same mode maintains its status, thus providing continuous oscillation. The ideal condition for MHF tuning is that the ratio of change of the cavity optical path length ${L_c}$ is equal to that of the etalon characteristic optical path length ${L_e}$, i.e.:
As it is shown in Fig. 1, the tunable laser cavity is composed of a laser diode, a cavity collimating lens, an etalon with a NBP filter, a periscope, and an end mirror. The periscope, the etalon and the NBP filter are mounted on a rotating stage, so that they can rotate together with the same rotating angle. The periscope comprises of two fully reflective mirrors to move the cavity modes in frequency by changing the cavity length.
Fig. 1. Design of the laser cavity with a periscope, etalon and NBP filter placed on a rotating stage.
Laser cavity optical length ${L_c}$ through the etalon and the periscope with an incident angle ${\theta _p}$ on the periscope mirror can be written as:
We denote d as periscope length and h as cavity height which is the single optical path excluding the effect from the periscope.
Therefore, this simplifies the cavity optical length ${L_c}$ as shown in Eq. (4). For a detailed derivation, refer to Section (1) of Supplement 1.
Considering the etalon as a F-P interferometer, the optical path difference between two neighboring rays is:
where t is the thickness of the etalon, $\alpha $ is the internal angle inside the etalon, m is the mode number, and we call ${L_e}$ as the etalon characteristic length. For a solid etalon, if n is the refractive index of the solid medium, ${\theta _e}$ is the light incident angle on etalon, then ${L_e}$ can be written in Eq. (6). For a full derivation refer to Section (1) of Supplement 1.Observing Eqs. (4) and (6), we can conclude that when cavity height h is equal to zero and an air gap etalon is adopted, the tracking condition as in Eq. (1) is satisfied for all angles (on the condition that the cavity length variation resulted from the rotation of etalon is neglected). The change of the cavity optical length would perfectly match the difference of the etalon characteristic length when the change of the incident angle of the periscope $\delta {\theta _p}$ and the etalon $\delta {\theta _e}$ are same. As the periscope and the etalon are rotated on the same stage with the same angular velocity, their rotation angle should be same, and $h = 0$ condition can be achieved by resetting the laser cavity as shown in Fig. 2.
In our practical design, a solid etalon is adopted instead of an air gap etalon because a solid etalon is much cheaper and can also provide enough tuning range (>4.8 THz) according to the simulation results shown in Fig. 3(b). To obtain the optimum relationship of periscope length d, cavity height h, periscope incident angle ${\theta _p}$ and etalon incident angle ${\theta _e}$ that gives a maximum tuning range, Eqs. (4) and (5) are inserted into Eq. (1). For approximation, Taylor-series expansion is applied on both sides of the equation, and an equation set is obtained by equating the first order term and the second order term of both sides (see in Supplement 1). Then we find that ${\theta _p}$ and ${\theta _e}$ are independent of periscope length d. In our design, ${\theta _p} = 14^\circ$ and $d = 16$ mm are chosen to avoid the clipping of the beam by the two periscope mirrors.
Fig. 3. (a) Optimal periscope incident angle and cavity height versus etalon incident angle (d = 16 mm). (b) MHF Tuning range for a wavelength of around 652 nm at different incident angles (d = 16 mm).
Figure 3(a) shows the relationship between the periscope incident angle and the etalon incident angle for BK7 glass (N-BK7 SCHOTT) etalon. It also shows the relationship between h and ${\theta _e}$ when $d = 16$ mm. The dash lines show that h is around 46.7 mm and ${\theta _p}$ is around 14.8° when ${\theta _e}$ is 14° chosen in our experimental design.
To check the maximum MHF tuning range for the designed periscope (i.e., ${\theta _p} = 14^\circ$ and $d = 16$ mm), we use the mode hop free tuning condition [1]:
This MHF tuning condition assumes that the change in mode number must be $< 1/2$ to avoid a mode hopping and the gain of the laser diode is the same for all the wavelength of consideration. Figure 3(b) shows the variation of the theoretical tuning range with an initial periscope angle for a periscope $d = 16$ mm and operation wavelength at ∼652 nm. The tuning range increases with the increase of initial periscope angle. For ${\theta _p} = 14^\circ $, the tuning range > 4.8 THz can be achieved. Careful design of the laser beam size to enlarge the incident angle can increase the MHF tuning range but etalon ‘walk-off’ effect must be considered [51].
3. ECDL configuration and device characteristics
The configuration of our ECDL and a photograph of the experimental structure are shown in Fig. 4(a) and 4(b), respectively. Laser diode with a curved section of gain chip is used with anti-reflection (AR) coating <0.1% on one facet and high reflection (HR) coating >99.5% on the other facet. To allow the output of the ECDL, a folding mirror is used to give the space for the end mirror with a focus lens and output lens shown in the Fig. 4(a). This configuration is slightly different with the ideal design shown in Fig. 1 which has the folding mirror in the cavity, however, the tuning optimum relationship is independent of the folding angle found in the approximation solution in section 2. The etalon and the NBP filter are combined by coating the thin film onto the etalon and attached on the top of one of the periscope mirrors. The folding mirror has been placed to bend the light output to the direction of the output of the gain chip. The extended laser cavity is composed of one facet of the gain chip with high reflection and the partially reflective end mirror. The light path is shown as red lines in Fig. 4(a). The total optical cavity length is 153 mm with a cavity folding angle of 8°, and a periscope angle of 14°. The details of the optical elements have been summarized as below.
The laser diode (gain chip, AlGaInP/GaAs multi-quantum well) has a ridged waveguide length of 500 µm and width of 3 µm with a curved section with radius of curvature of 2391.7 µm at facet angel of 6°, which is designed to avoid the reflection from the facet to suppress the gain ability to lase. The collimation lens (Geltech 354105) has an AR coating on both sides with a focal length of 5.5 mm, numerical aperture (NA) of 0.564, and clear aperture of 6.0 mm (aspherical side) and 4.96 mm (plano side). The lens is made of D-ZK7 with a central thickness of 2.937 mm. The etalon (BK7 glass) used in the configuration has a thickness of 400 µm and finesse about 6 with reflectivity of 60% on both sides, and an optical window of 9.0 mm × 7.2 mm. Flatness of λ/10 wavelength over any 6 mm diameter spot size within the optical aperture can be achieved with polarization insensitive coating for angle of incidence (AOI) of 14°±2° and wavelength range of 650 ± 20 nm.
A NBP filter is designed with full width half maximum (FWHM) bandwidth of 0.8 nm for AOI of 14°±2° at a beam waist radius of 2 mm. The transmission wavelength peak for AOI of 14° is within the range of 652 ± 1 nm; For AOI of 14°, the transmission peak of the bandpass filter is aligned to one of the transmission peaks of the etalon (this transmission peak of the etalon is denoted as the selected peak of the etalon) within the range of 652 ± 1 nm with alignment accuracy better than ±0.1 nm. The transmission of the bandpass at the selected peak of the etalon is >95%.
At the other transmission peaks of the etalon, the transmission for AOI of 14° through the bandpass is at least 5% lower than that at the selected peak of the etalon which can effectively suppress the side modes to allow single longitudinal mode operation. Furthermore, the NBP filter has an effective refractive index that matches the counterpart of the etalon (BK7) to be within ±0.01. The thermal coefficient that has been considered is that the optical path change of the NBP filter is different to the etalon by at least 3 ppm/K (note that the filter is coated on to the etalon so that they could be tuned synchronously). The periscope is designed with $d = 16$ mm with nominal incident angle of 14°. Two mirrors parallel tolerance is <1.0 arcsecond. The folding mirror has reflectivity >99.5% with lateral size of 13 mm × 9 mm and optical window of 9.0 mm × 9.0 mm. Focus lens and output lens (Geltech 354105) has AR coated at red band. The end mirror (12 mm × 7.2 mm) has reflectivity of 85% ± 5% to allow the laser output with enough cavity gain.
The output spectrum of the NBP filter, etalon, and the combination of two with respect to angles are shown in Fig. 5(a), (b), and (c), respectively. The spectrum shows a wavelength change of approximately 1.2 nm per degree of rotation for the NBP filter in Fig. 5(a). Within the etalon, the light experiences multiple reflection between the mirrored surfaces. This interference in relation to incident angle and etalon thickness performs constructive or destructive interference, which in turn creates a narrow peak at desired wavelengths shown in Fig. 5(b).
Fig. 5. Spectrum with incident angle from 13° to 15° for (a) single NBP filer, (b) single etalon, and (c) combination of them.
The effect of a combined NBP filter with etalon provides better stability as a singular component reduces tolerance issues and minimizes movement of the etalon in respect to the filter. The peak wavelength of the filter for optimum tracking is required to be in phase with one of the etalon peaks. Etalon and NBP filter production is provided by Alluxa in Santa Rosa, USA. The output spectrum of the combined filter with etalon in respect to incident angle can be seen in Fig. 5(c). The tuned wavelengths are clearly defined with a single mode of operation. The side peaks are passive, and the dominant wavelength will perform the mode tracking for the external cavity. Especially, we can see that single dominant peaks are captured under the range of AOI from 13° to 15° due to the perfect synchronization of the filter and etalon peak.
4. Experimental results
4.1 MHF tuning range
The experimental setup to measure the MHF tuning range is shown in Fig. 6. The details of the Spectrometer and the FP interferometer are shown below. The spectrometer is a high-resolution Ocean Optics HR4000 with a wavelength range from 620 nm to 675 nm and resolution <0.02 nm. The scanning FP interferometer is a Thorlabs SA210-5B with a wavelength range from 525 nm to 820 nm, FSR range of 10 GHz, finesse of 150 (180 typical), and resolution of 67 MHz. A maximum beam diameter of 150 µm is allowed through the cavity composed of two UV fused silica mirrors with a cavity length of 7.5 mm.
Two types of mode hopping mechanisms are observed during experiment: i) hops between etalon modes, and ii) hops between cavity modes within the same etalon mode. The spectrometer is used to observe the first type of mode hopping, meanwhile the scanning F-P interferometer is used to observe the second type of mode hopping, as the FSR (10 GHz) of the scanning F-P interferometer is shorter than the etalon peak period (∼257 GHz or ∼0.36 nm), and the cavity mode space (∼1 GHz or ∼0.001 nm) is shorter than the resolution of the spectrometer.
As shown in Fig. 7(a), (b) and (c), the spectrum of the different types of mode hopping are monitored by the spectrometer and the inserted images show the oscilloscope monitoring from the scanning F-P interferometer at the same time. Two traces are shown in the oscilloscope screen, the top trace is the periodical saw-tooth trigger signal, and the bottom trace plots the transmitted intensity through the F-P interferometer. Within each trigger period there are approximate two FSRs.
Fig. 7. Various type of mode hopping (a) no mode hopping, (b) mode hopping between two etalon peaks, and (c) mode hopping between cavity modes within an etalon peak. (d) Superimposed optical spectra of the MHF tuning.
Figure 7(a) illustrates the situation when there is no mode hop present, where only one peak is observed on the spectrometer. On the oscilloscope screen, there are two identical peaks in the transmitted intensity signal within each trigger period. In Fig. 7(b), there are two peaks on the spectrometer’s spectrum while two or more peaks in each FSR of the scanning F-P interferometer can be found. The variation of the signal shape from one FSR to the next shows the transit phenomenon. This phenomenon can be considered as mode hopping between two etalon peaks. The spectrum for the situation when the mode hops exist between two cavity modes inside an etalon peak is shown in Fig. 7(c). In this case, the spectrometer can only detect one peak while two peaks in each FSR can be found by F-P interferometer detection.
Figure 7(d) shows the superimposed optical spectra for the MHF tuning. The wavelength shown on the spectrometer is continuously altering while the wave crests passing on the F-P interferometer are estimated (see Visualization 1). As the FSR of the F-P interferometer is 10 GHz, the MHF tuning over 1.7 THz (∼2.4 nm) has been observed at the central wavelength of about 652 nm when rotating the angle between 13° and 15°. The rotational stage used in the experiment is the Newport SR50CC. The tuning capabilities of this stepper motor has the angular resolution of 0.001° limited by the step resolution of a DC Servo motor.
4.2 Laser linewidth
A scanning F-P interferometer from Coherent is used on the initial observation of laser linewidth. Figure 8 shows a screen print from the oscilloscope. The top red curve shows the transmitted intensity of the adjacent longitudinal cavity modes and the distance between the two peaks (FSR) are 600 MHz. Using the measuring tools on the oscilloscope, we further estimate the linewidth (FWHM) of the laser is <10 MHz.
To measure the accuracy and observation of the laser linewidth, an unbalanced Michelson interferometer is built up as shown in Fig. 9. The laser output is converted from linear polarized to circularly polarized light by employing a quarter wave plate. The circularly polarized beam is then divided into two linearly polarized beams by the polarizing beam splitter. Path A is relatively short, whereas Path B has been extended to achieve the longer distance delay. The return beams can be directed to the Renishaw ML10 unit and observed on the oscilloscope.
The ML10 detection unit gives two intensity signals: ${I_x} = a + b\cos \varphi $ and ${I_y} = a + b\sin \varphi $, where a and b are constant and $\varphi $ is the phase. Considering the relation between the delay phase $\varphi $ and laser frequency $\upsilon $, $\varphi = 2\pi \cdot 2{n_0}D\cdot \upsilon /c$, where c is the speed of light in air, ${n_0}$ is the refractive index of air (${\approx} 1$), and D is the interferometer arm length difference. The differential phase variation is from the laser frequency variation. Therefore, from the differential phase fluctuation $\Delta \varphi $, the laser frequency fluctuation or laser linewidth defined as differential frequency fluctuation $\Delta \upsilon $ can be expressed as:
Two intensity signals are connected to the ML10 and then to the oscilloscope with an interferometer arm length difference D. The classic Lissajous pattern indicates the phase difference by the shape of the X-Y plot. An approximate line shape is observed which indicates a phase difference of around $\pi $ when D is around 75 m. Therefore, the laser linewidth is estimated to be <1 MHz according to the Eq. (8). Note that the laser linewidth is expected to be narrower when the end mirror assembly is fixed instead of hanging on the adjustment jigs, as the vibration could affect the measured laser linewidth to be much larger. Furthermore, 0.1 mm difference of the cavity height h can cause tuning range drop from 4 THz to 1 THz.
4.3 Laser power and degradation
The output power of the laser is measured by LaserMet AMD 1000 power meter. Figure 10 shows the output power of the ECDL when operating at 65 mA drive current and 23 °C control temperature. The angle corresponds to the deviation from the designed incident angle of the periscope at 14°. The output power increases from 0.85 mW to ∼1 mW when the periscope is tuned from 12° to 14°, then decreases to ∼0.83 mW when the periscope is tuned continuously from 14° to 16°. The output power also shows quasi-periodical oscillation, in which the small period is due to the residue reflection from the output facet of the laser diode. The two circles in Fig. 10 illustrate the positions of the power jumping caused by mode hopping. For the current laser design, 1 degree of variation of incident angle can give a ∼1 THz of tuning range. The power variation is less than 10% in the region from 13° to 15°. Therefore, when the center of the MHF tuning region is set at the center of the power variation peak, a 1.7 THz MHF tuning range with power variation <10% has been achieved.
The output power of the ECDL without tuning is also monitored, where multiple longitudinal modes of operation is observed from the scanning F-P interferometer shown as the insert in Fig. 10, where the relative intensities change rapidly. This is due to the degradation of the laser diode which happens after more than 100 hours of running at a relatively high injection current of 65 mA (control temperature ∼23 °C). The degradation may be led by i) the contamination of the gain chip waveguide or the end facets of the laser diode, or ii) inhomogeneity on the coating of the end facets. Figure 11 shows the light output-current (L-I) characteristics of the laser. Laser output power of around 1 mW is obtained when the laser turning is measured at the injection current of 40 mA. The laser device shows stability under a constant operating (over 12 hours) with no more than 10% power fluctuation and the laser wavelength shifts are seldom observed.
5. Conclusion
A novel periscope tuning scheme for MHF tunable laser has been demonstrated in this study. This periscope tuning scheme can achieve an ideal synchronization between the cavity mode and etalon mode when using a very thin edged air-gap etalon. Although the periscope with a solid etalon tuning scheme cannot achieve the ideal tracking, it still provides a very large tuning range (>4.8 THz at incident angle of 14°) theoretically. We utilize a solid etalon instead of air one in our experiment and a tuning range of 1.7 THz has been demonstrated with power variation <10% at the wavelength of 652 ± 2 nm. The difference between the theoretical and experimental tuning range may be caused by the deviation of positioning the optical components, especially the end mirror. The output linewidth for the ECDL is estimated to be <1 MHz, observed using a scanning Fabry-Perot (F-P) and a Michelson interferometer. The laser linewidth should be narrower when all the optical components are carefully assembled and fixed especially the output lens with fiber coupling out. The self-heterodyne method could be used to achieve more accurate laser linewidth measurement in further work. Obviously, the new design has coarser mechanical movement tolerance due to abandoning the pivot-point-based grating tuning. MHF tuning condition only depends on the incident angle when other parameters are optimized and insensitive to the displacement of the moving parts. Therefore, the service life of this MHF tunable ECDL is ideally dependent on the lifetime of the laser diode instead of the mechanical parts abrasion which could be estimated in the forthcoming study. Further works could also be carried out to improve the tuning range and speed with the design using roof top mirrors and double side reflecting mirrors such as MEMS. This platform is highly promising to provide a high-speed broadband MHF tuning laser source with longer lifetime and very low cost for applications such as FSI, stimulated Raman spectroscopy (SRS), frequency modulated continuous wave (FMCW) coherent laser radar and swept source optical coherence tomography (SS-OCT).
Funding
Engineering and Physical Sciences Research Council (EP/I029613/1).
Acknowledgments
The authors thank Dr. Jungang Huang, Dr. Daniel Rees-Whippey, Dr. Christopher Price, Dr. Peter Rees, Dr. Yongkang Gong, Dr. Yuanlong Fan and Prof. Yani Zhang for useful discussions.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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